Both にしては and わりには allow us to make statements about things in an objective manner.

He looks young for his age.
The test score was poor for him.

The difference between the two phrases is subtle but one that means にしては cannot be used with adjectives. First compare these example sentences:

The quality is good given that it’s cheap.
The quality is good given that it’s cheap.

The difference is that にしては requires a mutual understanding between the speaker and listener which adjectives by their very nature cannot provide. “Cheap” is subjective—what is considered cheap by you might not be to me. 100 yen, however, is simple a fixed value. Consequently, both structures are acceptable in the below examples.

For 100 yen the quality is good.
For 100 yen the quality is good.

In terms of nuance, にしては emphasises a feeling of surprise on behalf of the speaker at the outcome whereas わりには is more neutral.

He’s pretty small for a sumo wrestler.
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